Marciano was knocked down twice (that I know of). In his fight against Walcott (when Rocky won the title) Marciano was knocked down in the first round and he was way behind in points when he finally knocked Jersey Joe out in the 13th round. (Walcott was almost 40 years old at that time and the toll of the fight tired him out enough for Marciano to finish him off - if it was a 12 rounder Walcott would have kept the title).

Archie Moore also knocked Marciano down (in the second round) in Rocky's last fight. Ezzard Charles had Marciano on the ropes early on in their second fight and the fight was almost stopped when Marciano's nose was split open.

Perhaps, but Marciano took a good deal of punishment in addition to giving it out and that counts for something. He was, IMO, the hardest hitting fighter of all time. His record of 88% of wins by KO (43 of 49 fights) is testament to that. He was a better fighter than you're giving him credit for.

If Ali and Marciano fought each other in their primes, Ali would have outboxed him but the fight would have gone the distance and Ali would have won only because of his superior physical being.